I answered yes to the auto-complete scripts question during opam init. Also, the truncated portion of the error suggests the following:

Please install a system-wide version of camlp4. This is necessary
since you are using OCaml 4.02.1 and the system compiler switch (which
uses your preexisting installation rather than compile one from
scratch).

If you cannot install a system-wide camlp4, then you can still use
OCaml
4.02 by switching to a local installation via opam switch 4.02.1

Answers

You should switch from a system compiler to an opam's one. Then the compiler's sources will be downloaded and compiled automatically as well as all prerequisites. In order to switch use the following command:

opam swich 4.02.1

instead of 4.02.1 you can use any available compiler of course.

The actual error, is that for some reason (I think due to a rolling release model) you have different versions of camlp4 and OCaml compiler, so that they can't talk to each other.